T4 and T3 are largely bound to plasma proteins in the circulation. TBG has the greatest affinity for T4 and T3, although a small proportion of the hormones also binds to prealbumin and to a lesser extent, to albumin. Albumin has a low affinity, but a large capacity, for T4 and T3. Measurement of circulating total thyroid hormone concentrations includes both protein-bound and free fractions of the hormones (Fig. 15.11), although the free fractions represent a tiny proportion (0.04% in the case of T4) of the total. The normal concentration of the free fraction of T4 lies between 9 and 24pmol/l (0.7 and 1.9ng/100ml), whereas the total T4 concentration lies between 50 and 140nmol/l (4-11mg/100ml). The normal concentration of free T3 lies between 5 and 10pmol/l (0.33 and 0.65ng/100ml), and that of total T3 between 1 and 3nmol/l (65 and 195ng/100ml). The level of free hormones governs the metabolic state and thyroid status of a patient.
